Constant slope ramp circuits for sampled-data circuits

ABSTRACT

A circuit includes a level-crossing detector to generate a level-crossing detection signal when an input signal crosses a predetermined voltage level. A first stage set of capacitors is operatively coupled to the level-crossing detector. A ramp circuit is operatively coupled to the set of series-connected capacitors. A second stage set of capacitors is operatively coupled to the first stage set of capacitors and the ramp circuit. The ramp circuit includes a feedback capacitor and a preset switch to provide a linear ramp output.

PRIORITY INFORMATION

This application is a divisional of and claims priority benefit of commonly-assigned U.S. patent application Ser. No. 12/484,489, titled “Constant Slope Ramp Circuits for Sampled Data Circuits,” filed on Jun. 15, 2009, listing inventor Hae-Seung Lee, now U.S. Pat. No. 8,294,495, which patent is hereby incorporated herein by reference in its entirety.

Said co-pending U.S. patent application Ser. No. 12/484,489 is related to and claims the priority benefit of U.S. patent application Ser. No. 11/488,550, now issued as U.S. Pat. No. 7,737,732, titled “Constant slope ramp circuits for sample-data circuits,” filed on Jul. 18, 2006, listing inventor Hae-Seung Lee, which application is incorporated by reference in its entirety herein.

Said U.S. patent application Ser. No. 11/488,550 is related to and claims the priority benefit of U.S. patent application Ser. No. 11/454,275, filed on Jun. 16, 2006, titled “Sampled-data circuits using zero crossing detection,” now issued as U.S. Pat. No. 7,486,115, listing inventor Hae-Seung Lee, which application is incorporated by reference in its entirety herein.

Said U.S. patent application Ser. No. 11/488,550, filed on Jul. 18, 2006 claims priority, under 35 U.S.C. §119(e), from U.S. Provisional patent application Ser. No. 60/595,605, filed on Jul. 19, 2005. Said U.S. patent application Ser. No. 11/488,550, filed on Jul. 18, 2006 also claims priority, under 35 U.S.C. §119(e), from U.S. Provisional patent application Ser. No. 60/595,623, filed on Jul. 21, 2005.

Thus, the present application also claims priority, under 35 U.S.C. §119(e), from U.S. Provisional patent application Ser. No. 60/595,605, filed on Jul. 19, 2005, and claims priority, under 35 U.S.C. §119(e), from U.S. Provisional patent application Ser. No. 60/595,623, filed on Jul. 21, 2005.

Said U.S. patent application Ser. No. 11/454,275, filed on Jun. 16, 2006 claims priority, under 35 U.S.C. §119(e), from U.S. Provisional patent application Ser. No. 60/595,414, filed on Jul. 1, 2005. Said U.S. patent application Ser. No. 11/454,275, filed on Jun. 16, 2006 also claims priority, under 35 U.S.C. §119(e), from U.S. Provisional patent application Ser. No. 60/595,493, filed on Jul. 11, 2005.

Thus, the present application also claims priority, under 35 U.S.C. §119(e), from U.S. Provisional patent application Ser. No. 60/595,414, filed on Jul. 1, 2005, and claims priority, under 35 U.S.C. §119(e), from U.S. Provisional patent application Ser. No. 60/595,493, filed on Jul. 11, 2005.

The contents of U.S. patent application Ser. No. 12/484,489, filed on Jun. 15, 2009; U.S. patent application Ser. No. 11/488,550, filed on Jul. 18, 2006; U.S. patent application Ser. No. 11/488,548, filed on Jul. 18, 2006; U.S. patent application Ser. No. 11/454,275, filed on Jun. 16, 2006; U.S. Provisional patent application Ser. No. 60/595,414, filed on Jul. 1, 2005; U.S. Provisional patent application Ser. No. 60/595,493, filed on Jul. 11, 2005; U.S. Provisional patent application Ser. No. 60/595,605, filed on Jul. 19, 2005; and U.S. Provisional patent application Ser. No. 60/595,623, filed on Jul. 21, 2005 are hereby incorporated by reference in their entirety.

BACKGROUND Background of the Invention

Most sample-data analog circuits such as switched-capacitor filters, analog-to-digital converters, and delta-sigma modulators require operational amplifiers to process the signal. Consider a switched-capacitor integrator example shown in FIG. 2. First, the switches S₁₁ and S₁₃ are closed so that the input voltage v_(in) is sampled on the sampling capacitor C_(S1). Next, the switches S₁₁ and S₁₃ are opened and S₁₂ and S₁₄ are closed. This operation transfers the charge in the sampling capacitor C_(S1) to the integrating capacitor C₁₁. The output voltage, v_(out), of a first integrator 1100 is typically sampled by another sample-data circuit, for example, another switched-capacitor integrator.

In the circuit shown in FIG. 2, the circuit consisting of switches S₂₁, S₂₂, S₂₃, S₂₄, and a second sampling capacitor C_(S2) comprise a part of the second switched-capacitor integrator. The output voltage, v_(out), of the first integrator 10 is sampled on the second sampling capacitor C_(S2) by closing switches S₂₁ and S₂₃.

An example of a timing diagram is shown in FIG. 3. The clock signal has two non-overlapping phases φ₁ and φ₂. The phase φ₁ is applied to switches S₁₁, S₁₃, S₂₁, and S₂₃, and phase φ₂ is applied to switches S₁₂, S₁₄, S₂₂, and S₂₄. With this timing, the circuit performs non-inverting discrete integration with full clock delay. The waveforms at the output of the integrator, v_(out), and at the virtual ground node 100, v₁, are also shown in FIG. 3. Different clock phasing arrangements yield different responses from the integrator. For example, if φ₁ is applied to switches S₁₁, S₁₃, S₂₂, and S₂₄, and phase φ₁ is applied to switches S₁₂, S₁₄, S₂₁, and S₂₃, the circuit performs non-inverting integration with half-clock delay.

For an accurate integration of the input signal, v₁ must be driven as close to ground as possible. In order to accomplish this, the operational amplifier must provide sufficient open-loop gain and low noise. In addition, for fast operation, the operational amplifier 10 of FIG. 2 must settle fast.

In FIG. 3, the voltage v₁ is shown to settle back to ground after a disturbance when the sampling capacitor C_(S1) is switched to Node 100 by closing S₁₂ and S₁₄. In addition to high open-loop gain and fast settling time, operational amplifiers must provide large output swing for high dynamic range. As the technology scales, it becomes increasingly difficult to achieve these characteristics from operational amplifiers. The primary factors that make the operational amplifier design difficult are low power supply voltages and low device gain.

As noted above, accurate output voltage can be obtained if Node 100 in FIG. 2 is maintained precisely at ground. However, in sample-data circuits, the only point of time accurate output voltage is required is at the instant the output voltage is sampled by another sampling circuit. Thus, it is not necessary to maintain the voltage at Node 100 at ground all the time.

It is further noted that delays in the detectors of sample-data circuits may cause errors in the operation thereof. Moreover, it is noted that performance parameters such as speed, accuracy, and power consumption of sample-data circuits depend upon the design of zero crossing detectors.

Therefore, it is desirable to provide a sample-data circuit that maintains the proper level at the virtual ground node at the instant the output voltage is sampled by another sampling circuit, substantially eliminates the errors caused by delays, and optimizes the performance parameters such as speed, accuracy, and power consumption of the sample-data circuit.

Moreover, it is desirable to provide a sample-data circuit that maintains the proper level at the virtual ground node at the instant the output voltage is sampled by another sampling circuit and provides differential signal paths for sample-data circuits, substantially eliminates the errors caused by delays, and optimizes the performance parameters such as speed, accuracy, and power consumption of the sample-data circuit.

Furthermore, it is desirable to provide a sample-data circuit that reduces the effect of power supply, substrate, and common-mode noise by symmetric differential signal processing, substantially eliminates the errors caused by delays, and optimizes the performance parameters such as speed, accuracy, and power consumption of the sample-data circuit.

Also, it is desirable to provide a sample-data circuit that increases the signal range by incorporating differential signal paths, substantially eliminates the errors caused by delays, and optimizes the performance parameters such as speed, accuracy, and power consumption of the sample-data circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to embodiments of the invention, examples of which may be illustrated in the accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in the context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.

FIG. 1 illustrates a zero-crossing detector;

FIG. 2 illustrates a switched-capacitor integrator;

FIG. 3 illustrates a timing diagram for the switched-capacitor integrator of FIG. 2;

FIG. 4 illustrates a non-inverting integrator according to the concepts of the present invention;

FIG. 5 illustrates a timing diagram for the non-inverting integrator of FIG. 4;

FIG. 6 illustrates a non-inverting integrator with a waveform generator being a current source according to the concepts of the present invention;

FIG. 7 illustrates another non-inverting integrator according to the concepts of the present invention;

FIG. 8 illustrates a timing diagram for the non-inverting integrator of FIG. 7;

FIG. 9 illustrates another non-inverting integrator according to the concepts of the present invention;

FIG. 10 illustrates another non-inverting integrator according to the concepts of the present invention;

FIG. 11 illustrates a timing diagram for the non-inverting integrator of FIG. 10;

FIG. 12 illustrates another non-inverting integrator according to the concepts of the present invention;

FIG. 13 illustrates another non-inverting integrator according to the concepts of the present invention;

FIG. 14 illustrates a timing diagram for the non-inverting integrator of FIG. 13;

FIG. 15 illustrates one embodiment of a sample-data circuit according to the concepts of the present invention;

FIG. 16 illustrates another embodiment of a sample-data circuit according to the concepts of the present invention;

FIG. 17 illustrates another embodiment of a sample-data circuit according to the concepts of the present invention;

FIG. 18 illustrates another embodiment of a sample-data circuit according to the concepts of the present invention;

FIG. 19 illustrates another embodiment of a sample-data circuit according to the concepts of the present invention;

FIG. 20 illustrates an embodiment a continuous-time voltage comparator for a sample-data circuit according to the concepts of the present invention;

FIG. 21 illustrates another embodiment a continuous-time voltage comparator for a sample-data circuit according to the concepts of the present invention;

FIG. 22 illustrates another embodiment a continuous-time voltage comparator for a sample-data circuit according to the concepts of the present invention;

FIG. 23 illustrates another embodiment a continuous-time voltage comparator for a sample-data circuit according to the concepts of the present invention;

FIG. 24 illustrates another embodiment a continuous-time voltage comparator for a sample-data circuit according to the concepts of the present invention;

FIG. 25 illustrates another embodiment a continuous-time voltage comparator for a sample-data circuit according to the concepts of the present invention;

FIG. 26 illustrates another embodiment a continuous-time voltage comparator for a sample-data circuit according to the concepts of the present invention;

FIG. 27 illustrates another embodiment a continuous-time voltage comparator for a sample-data circuit according to the concepts of the present invention; F

FIG. 28 illustrates another embodiment a continuous-time voltage comparator for a sample-data circuit according to the concepts of the present invention;

FIG. 29 illustrates a constant slope ramp circuit;

FIG. 30 illustrates an example of a 1-bit per stage pipeline analog-to-digital converter with a conventional ramp circuit;

FIGS. 31 and 32 illustrate capacitor configurations of zero-crossing based circuits during a charge-transfer phase and a sampling phase, respectively;

FIG. 33 illustrates a constant slope ramp circuit with multiple outputs; and

FIG. 34 illustrates an example of a 1-bit per stage pipeline analog-to-digital converter with a multiple output ramp circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For a general understanding of the present invention, reference is made to the drawings. In the drawings, like reference have been used throughout to designate identical or equivalent elements. It is also noted that the various drawings illustrating the present invention may not have been drawn to scale and that certain regions may have been purposely drawn disproportionately so that the features and concepts of the present invention could be properly illustrated.

It is noted that, in the various Figures, the earth symbol indicates the system's common-mode voltage. For example, in a system with 2.5 V and −2.5 V power supplies, the system's common-mode voltage may be at ground. In a system with a single 2.5 power supply, the system's common-mode voltage may be at 1.25 V.

As noted above, accurate output voltage can be obtained if Node 100 in FIG. 2 is maintained precisely at ground. However, in sample-data circuits, the only point of time accurate output voltage is required is at the instant the output voltage is sampled by another sampling circuit. Thus, it is not necessary to maintain the voltage at Node 100 at ground all the time.

FIG. 4 illustrates a non-inverting integrator according to the concepts of the present invention. More specifically, as an example, a non-inverting integrator with half-clock delay is illustrated in FIG. 4.

As illustrated in FIG. 4, a clock phase φ₁ is applied to switches S₁₁, S₁₃, S₂₂, and S₂₄, and another phase φ₂ is applied to switches S₁₂, S₁₄, and S₂₁. A zero crossing detector 30 is used to detect the point of time at which Node 100 crosses ground. The switch S₂₃ is controlled by the output of the zero crossing detector 30. The output of the zero crossing detector 30 is used to determine the time point to take the sample of the output voltage v_(out). A waveform generator 20 generates a voltage waveform as the output voltage v_(out) in such way the voltage at Node 100 crosses zero if the charge in capacitors C_(S1) and C₁₁ is within a normal operating range.

In the timing diagram shown in FIG. 5, the waveform generated by the waveform generator 20 is shown as a ramp. When v₁, the voltage at Node 100, crosses zero at time t₁, the output v_(zc) of the zero crossing detector 30 goes low, turning the switch S₂₃ OFF. At that instant, the output voltage v_(out) is sampled on C_(S2).

Since v₁ is very close to zero when the sample of v₂ is taken, an accurate output voltage is sampled on C_(S2). A similar operation repeats during the next clock cycle, and the sample of the output voltage is taken at time t₂.

It is noted that the zero crossing detector 30 may optionally have an overflow detection feature that determines when the charge in capacitors Cs₁ and C_(I1) is outside the normal range of operation. It can be implemented by a logic circuit that makes the output v_(zc) of the zero-crossing detector 30 to go low when φ₂ goes low.

In the event v₁ fails to cross zero, the sample is taken on the falling edge of φ₂. At the same time, the logic circuit produces a flag indicating overflow.

In the embodiment described above and in the various embodiments described below, a zero crossing detector is utilized in lieu of a comparator. Typically, a comparator is designed to compare two arbitrary input voltages. A comparator may be implemented as cascaded amplifiers, a regenerative latch, or a combination of both. A comparator may be used to detect a zero voltage level or a predetermined voltage level crossing.

It is noted that the input waveform of the various described embodiments is not arbitrary, but deterministic and repetitive. Thus, the various described embodiments determine the instant the zero voltage level or the predetermined voltage level is crossed than relative amplitudes of the input signals. For such a deterministic input, a zero crossing detector is more efficient.

An example of a zero-crossing detector for the detection of a positive-going input signal is shown in FIG. 1. Initially, node 1 and node 2 are precharged to v_(DD) and ground, respectively. The ramp input voltage v_(in) is applied according to the zero crossing circuit. At the time the input node crosses the threshold, node 1 is discharged rapidly, and node 2 is pulled up to v_(DD). Since the zero crossing detector in FIG. 1 is a dynamic circuit, there is no DC power consumption, allowing extremely low power and fast operation. For the detection of zero-crossing of a negative-going signal, a complementary circuit with a PMOS input transistor can be utilized.

As illustrated in FIG. 6, the non-inverting integrator includes a waveform generator which is a current source 200. As illustrated in FIG. 6, a clock phase φ₁ is applied to switches S₁₁, S₁₃, S₂₂, and S₂₄, and another phase φ₂ is applied to switches S₁₂, S₁₄, and S₂₁. A zero crossing detector 30 is used to detect the point of time at which Node 100 crosses ground. The switch S₂₃ is controlled by the output of the zero crossing detector 30. The output of the zero crossing detector 30 is used to determine the time point to take the sample of the output voltage v_(out).

The current source 200 charges the capacitors C_(S2) and the series connected C_(S1) and C_(I1), generating a ramp. At the start of φ₂, the output is briefly shorted to a known voltage V_(NEG), the value of which is chosen to ensure the voltage v₁ at Node 100 crosses zero with signals in the normal operating range.

As illustrated in FIG. 7, the non-inverting integrator includes a waveform generator 20 that produces, preferably, a plurality of segments in the waveform with varying rate of change of the output voltage. The first segment may be controlled so as to have the highest rate of change, with subsequent segments having progressively lower rate of change. The detection of zero crossing by the zero crossing detector 30 causes the waveform to advance to the next segment. An output signal v_(zc2) of the zero crossing detector 30 remains high until the zero crossing is detected in the last segment of the waveform

One clock cycle of the timing diagram is shown in FIG. 8. At the start of φ₂, the waveform generator 20 produces an up ramp. The voltage v₁ is shown to cross zero at time t₁. One output, v_(zc1), of the zero crossing detector 30 changes its state after a finite delay t_(d1).

The delay t_(d1) represents finite delay of a typical zero crossing detector 30. This change of state advances the waveform to the next segment.

Due to the delay t_(d1) of the zero crossing detector 30, the voltage v₁ overshoots by a small amount above ground. The second segment of the waveform generator is a down ramp to permit another zero crossing at time t₂. After a second delay t_(d2), the output v_(zc2) of the zero crossing detector 30 goes low, causing the switch S₂₃ to turn OFF, locking the sample of the output voltage v_(out).

The delay t_(d2) of the second zero crossing is not necessarily the same as the delay associated with the first zero crossing t_(d1). The delay t_(d2) contributes a small overshoot to the sampled output voltage. The effect of the overshoot can be shown to be constant offset in the sampled charge. In most sample-data circuits, such constant offset is of little issue.

The zero crossing detector 30 preferably becomes more accurate in detecting the zero crossing as the segments of the waveform advances. The first detection being a coarse detection, it doesn't have to be very accurate. Therefore, the detection can be made faster with less accuracy. The last zero crossing detection in a given cycle determines the accuracy of the output voltage. For this reason, the last zero crossing detection must be the most accurate.

The accuracy, speed, and the power consumption can be appropriately traded among progressive zero crossing detections for the optimum overall performance. For example, the first detection is made less accurately and noisier but is made faster (shorter delay) and lower power. The last detection is made more accurately and quieter while consuming more power or being slower (longer delay).

An example of a two-segment waveform generator constructed of two current sources (210 and 220) is shown in FIG. 9. As illustrated in FIG. 9, a clock phase φ₁ is applied to switches S₁₁, S₁₃, S₂₂, and S₂₄, and another phase φ₂ is applied to switches S₁₂, S₁₄, and S₂₁. A zero crossing detector 30 is used to detect the point of time at which Node 100 crosses ground. The switch S₂₃ is controlled by the output of the zero crossing detector 30. The output of the zero crossing detector 30 is used to determine the time point to take the sample of the output voltage v_(out).

Current sources 210 and 220 charge the capacitors C_(S2) and the series connected C_(S1) and C_(I1) generating two segments of a ramp waveform. At the start of φ₂, the output is briefly shorted to a known voltage v_(NEG), the value of which is chosen to ensure the voltage v₁ crosses zero with signals in the normal operating range. During the first segment, the current source 210 is directed to the output, while during the second segment, the current source 220 is directed to the output, generating two different slopes of ramp.

As illustrated in FIG. 10, the non-inverting integrator includes a level crossing detector 300 having plurality of thresholds. As illustrated in FIG. 10, a clock phase φ₁ is applied to switches S₁₁, S₁₃, S₂₂, and S₂₄, and another phase φ₂ is applied to switches S₁₂, S₁₄, and S₂₁. A level crossing detector 300 is used to detect the point of time at which Node 100 crosses one of plurality of predetermined levels as discussed below. The switch S₂₃ is controlled by the output of the level crossing detector 300. The output of the level crossing detector 300 is used to determine the time point to take the sample of the output voltage v_(out).

The thresholds are predetermined voltage levels. The thresholds of the level crossing detector 300 can be adjusted to minimize overshoot.

For example, the threshold for the first detection may be made negative by a slightly smaller amount than the expected overshoot in the first segment. This minimizes the ramp-down time in the second segment. Also, the threshold for the second segment may be made more positive by the amount of the overshoot in the second segment in order to cancel the effect of the overshoot. Alternatively, the threshold for the first segment may be made more negative than the expected overshoot during the first segment. This permits the second segment to be a positive ramp rather than a negative ramp as shown in FIG. 11.

It is advantageous to make the detection during the last segment to be the most accurate detection. The accuracy of the detection during the last segment is made higher than during other segments. This can be achieved by making the delay longer or making the power consumption higher during the last segment.

As illustrated in FIG. 12, the non-inverting integrator includes a level crossing detector having two zero-crossing detectors, Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320). As illustrated in FIG. 12, a clock phase φ₁ is applied to switches S₁₁, S₁₃, S₂₂, and S₂₄, and another phase φ₂ is applied to switches S₁₂, S₁₄, and S₂₁.

Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320) are used to detect the point of time at which Node 100 crosses one of plurality of predetermined levels as discussed below. The switch S₂₃ is controlled by the output of the Zero Crossing Detector 2 (320). The output of the Zero Crossing Detector 2 (320) is used to determine the time point to take the sample of the output voltage v_(out).

The thresholds of the Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320) are selected to minimize overshoot. For example, the threshold for Zero Crossing Detector 1 (310) may be made negative by a slightly smaller amount than the expected overshoot in the first segment. This minimizes the ramp-down time in the second segment. Also, the threshold for Zero Crossing Detector 2 (320) may be made more positive by the amount of the overshoot in the second segment in order to cancel the effect of the overshoot. Alternatively, the threshold for Zero Crossing Detector 1 (310) may be made more negative than the expected overshoot during the first segment. This permits Zero Crossing Detector 2 (320) to be a positive ramp rather than a negative ramp.

In other words, Zero Crossing Detector 1 (310) makes a coarse detection, whereas Zero Crossing Detector 2 (320) makes a fine detection. Thus, it is advantageous to make Zero Crossing Detector 2 (320) to have a higher accuracy.

As illustrated in FIG. 13, the non-inverting integrator includes a level crossing detector having two zero-crossing detectors, Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320). As illustrated in FIG. 13, a clock phase φ₁ is applied to switches S₁₁, S₁₃, S₂₂, and S₂₄, and another phase φ₂ is applied to switches S₁₂, S₁₄, and S₂₁. Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320) are used to detect the point of time at which Node 100 crosses one of plurality of predetermined levels as discussed below. The switch S₂₃ is controlled by the output of the Zero Crossing Detector 2 (320). The output of the Zero Crossing Detector 2 (320) is used to determine the time point to take the sample of the output voltage v_(out).

Both detectors, Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320), have nominally zero thresholds. The detection thresholds are determined by voltages v_(tr1) and v_(tr2) applied to the inputs of Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320), respectively. Zero Crossing Detector 1 (310) makes a coarse detection, whereas Zero Crossing Detector 2 (320) makes a fine detection. Thus, it is advantageous to make Zero Crossing Detector 2 (320) to have a higher accuracy.

It is noted that the above-described embodiment may operate as a self-timed system. In this configuration, Rather than supplying constant frequency clock phases φ₁ and φ₂, the clock phases are derived from the outputs of Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320). FIG. 14 illustrates a self-timed operation.

As illustrated in FIG. 14, the end of the phase φ₂ is defined by the output of the detection during the last segment. The beginning of the clock phase φ₁ is defined by a short delay, such as logic delays, after the end of φ₂. The short delay is generally necessary to ensure non-overlapping clock phases. The end of the clock phase φ₁ is determined by the zero crossing detection of the previous stage or the following stage in the similar manner

It is noted that the various embodiments described above can be utilized in a pipeline analog-to-digital converter, an algorithmic analog-to-digital converter, a switched-capacitor amplifier, a delta-sigma modulator, or a self-timed algorithmic analog-to-digital converter.

It is further noted that the various embodiments described above have signal paths that are single-ended, thus, it is desirable to provide differential signal paths. The various embodiments described below provide differential signal paths

As noted above, it is desirable to substantially eliminate the errors associated with time delays. One solution to substantially eliminate the errors associated with time delays is to have the error in a form that can be easily managed.

For example, if a waveform generator produces a ramp whose slope is constant in each segment of the ramp, the error associated with the delays becomes a constant DC offset. A constant DC offset can be easily compensated or tolerated in most systems. However, simple waveform generators, for example, the one shown in FIG. 6, produce non-linear ramp signals. The nonlinearity in the ramp slope of the conventional sample-data circuits causes nonlinearity in the resulting sample-data circuit error. The non-linear error is difficult to compensate.

To address this problem, the present invention provides a circuit that generates ramp waveforms with plurality of linear segments of varying slopes whereby the slope is independent of the output voltage and load conditions in each segment of the ramp. By generating ramp waveforms with plurality of linear segments of varying slopes, the error is a plurality of constant DC offsets that can easily compensated for or tolerated.

Furthermore, to address this problem, the present invention provides a circuit that generates ramp waveforms with plurality of linear segments of varying slopes whereby the advancement to the next segment is controlled by an output of a zero crossing detector.

One embodiment of the present invention that provides a circuit that generates ramp waveforms with plurality of linear segments of varying slopes is illustrated in FIG. 15. As illustrated in FIG. 15, a sample-data circuit includes an amplifier 500 that preferably provides large open loop gain. A zero crossing detector 400 detects the condition when an input voltage v₁ crosses another input voltage v₂, at which time the output signal v_(zcd) changes, signaling the detection of zero-crossing.

It is noted that a continuous-time voltage comparator can be used as a zero crossing detector. It is further noted that the zero crossing detector 400 may have a non-zero threshold voltage, if desired. Thus, a voltage level detector can be employed instead.

As illustrated in FIG. 15, before the start of the first segment of a ramp waveform generation, a capacitor C is precharged to a start voltage V_(start) by turning a switch S₁ ON and connecting a switch S₂ to the upper position, the upper position being connected to start voltage v_(start). Next, the switch S₁ is turned OFF, and the switch S₂ is connected to the lower position. The capacitor C now provides negative feedback.

In a single-slope operation as shown in FIG. 5, current source I provides constant current. For a multi-slope operation as shown in FIGS. 8 and 11, the current source I is a variable current source, and set to one of a plurality of current levels I₁. The output voltage v_(out) is approximately a linear ramp given by v_(out)=V_(start)+I₁t/C. In one embodiment, the current source I is implemented by a resistor connected to a constant voltage. In another embodiment, the current source I is implemented by a MOS transistor.

If I₁ is positive, the resulting v_(out) is a positive-going ramp. In a multi-slope operation, when the output signal v_(zcd) of the zero crossing detector 400 changes at time t=t₁, the value of the current source I is changed to the next value I₂ in order to produce the next segment of the ramp. The output voltage v_(out) in the second segment is v_(out)=v_(start)+I₁t₁/C+I₂t/C.

A negative-going second segment of the ramp can be generated by making I₂ negative. More segments can be produced by changing the value of the current I. In this embodiment, a ramp waveform generator produces substantially constant slope within each segment regardless of the load condition.

FIG. 16 shows another embodiment of the present invention created with CMOS technology. The structure and the operation are similar to the embodiment illustrated and described with respect to FIG. 15.

As illustrated in FIG. 16, an amplifier includes MOS transistors M₁ and M₂. A zero crossing detector 400 detects the condition when an input voltage v₁ crosses another input voltage v₂, at which time the output signal v_(zcd) changes, signaling the detection of a zero-crossing.

It is noted that a continuous-time voltage comparator can be used as a zero crossing detector. It is further noted that the zero crossing detector 400 may have a non-zero threshold voltage, if desired. Thus, a voltage level detector can be employed instead.

As illustrated in FIG. 16, before the start of the first segment of a ramp waveform generation, a capacitor C is precharged by turning a switch S₁ ON and connecting a switch S₂ to the upper position, the upper position being connected to start voltage V_(start). This precharges the capacitor C to v_(start)-v_(bias1). The voltage v_(bias1) may be approximately the same as the gate voltage of M₁. Next, the switch S₁ is turned OFF, and the switch S₂ is connected to the lower position. The capacitor C now provides negative feedback. In a single-slope operation as shown in FIG. 5, current source I provides constant current. For a multi-slope operation as shown in FIGS. 8 and 11, the current source I is a variable current source, and set to one of the plurality of current levels I₁. The output voltage v_(out) is approximately a linear ramp given by V_(out)=V_(start)+I₁t/C. In one embodiment, the current source I is implemented by a resistor connected to a constant voltage. In another embodiment, the current source I is implemented by a MOS transistor.

If I₁ is positive, the resulting v_(out) is a positive-going ramp. In a multi-slope operation, when the output signal v_(zcd) of the zero crossing detector 400 changes at time t=t₁, the value of the current source I is changed to the next value I₂ in order to produce the next segment of the ramp. The output voltage v_(out) is v_(out)=v_(start)+I₁t₁/C+I₂t/C.

A negative-going second segment of the ramp can be generated by making I₂ negative. More segments can be produced by changing the value of the current I. In this embodiment, a ramp waveform generator produces substantially constant slope within each segment regardless of the load condition

FIG. 17 shows another embodiment of the present invention. As illustrated in FIG. 17, a sample-data circuit includes a differential amplifier 500 that provides, preferably, a large open-loop gain. Before the start of the ramp generation, the switch S₁ is closed, and a first value R₁ of the resistor R is selected.

By the negative feedback through the transistor M₁ and the resistor R, the voltage across the resistor R is maintained at v_(bias)−v_(SS). Thus, the current through the resistor is I₁=(v_(bias)−v_(SS))R₁, and the initial output voltage is v_(bias).

Next, the switch S₁ is opened. By the negative feedback through the transistor M₁, the capacitor C, and the resistor R, the voltage across R is still maintained at v_(bias)−v_(SS) and the current through the resistor R at I₁=(v_(bias)−v_(SS))/R₁. The capacitor C is charged by the current I₁, giving the output voltage v_(out)=V_(bias)+I₁t/C=V_(bias)+(V_(bias)−V_(SS))t−/R₁C.

In a multi-slope operation, when the output signal v_(zcd) of the zero crossing detector changes at time t=t₁, the value of the resistor R is changed to the next value R₂ in order to produce the next segment of the ramp. The output voltage v_(out) in the second segment is v_(out)=v_(bias)+(V_(bias)−V_(SS))t₁/R₁C+(V_(bias)−V_(SS))t₂/R₂C. More segments can be produced by further changing the value of the resistor R. In this embodiment, a ramp waveform generator produces substantially constant slope within each segment regardless of the load condition.

The embodiment of FIG. 17 can generate multiple segments of a positive-going ramp waveform. The ramp can be stopped by turning the switch S₂ ON, which cuts off the transistor M₁.

FIG. 18 shows a complimentary embodiment of FIG. 17. As illustrated in FIG. 18, a sample-data circuit includes a differential amplifier 500 that provides, preferably, a large open-loop gain. Before the start of the ramp generation, the switch S₁ is closed, and a first value R₁ of the resistor R is selected.

By the negative feedback through the transistor M₁ and the resistor R, the voltage across the resistor R is maintained at V_(bias)−V_(DD). Thus, the current through the resistor is I₁=(V_(bias)−V_(DD))/R₁, and the initial output voltage is V_(bias).

Next, the switch S₁ is opened. By the negative feedback through the transistor M₁, the capacitor C, and the resistor R, the voltage across R is still maintained at V_(bias)−V_(DD) and the current through the resistor R at I₁=(V_(bias)−V_(DD))/R₁. The capacitor C is charged by the current I₁, giving the output voltage V_(out)=V_(bias)+I₁t/C=V_(bias)+(V_(bias)−V_(DD))t−/R₁C.

In multi-slope operation, when the output signal V_(zcd) of the zero crossing detector changes at time t=t₁, the value of the resistor R is changed to the next value R₂ in order to produce the next segment of the ramp. The output voltage v_(out) in the second segment is V_(out)=V_(bias)+(V_(bias)−_(DD))t₁/R₁C+(V_(bias)−V_(DD))t₂/R₂C. Multiple segments of a negative-going ramp waveform can be produced by further changing the value of the resistor R. In this embodiment, a ramp waveform generator produces substantially constant slope within each segment regardless of the load condition

Both positive-going and negative-going segments can be generated by another embodiment, as illustrated by FIG. 19. As illustrated by FIG. 19, before the start of the first segment, switches S₁, S₂, S₃, and S₅ are closed, and S₄ is opened. This configuration precharges a capacitor C₁ at v_(start)−v_(bias) 1 and another capacitor C₂ at v_(start)−v_(bias) 2, setting the initial condition of the output voltage at V_(start).

A first value R₂(1) of the resistor R₂ is selected. By the negative feedback through the transistor M₂ and the resistor R₂, the voltage across R₂ is maintained at v_(bias2)−v_(SS). Thus, the current through the resistor is given by I₁=(V_(bias2)−V_(SS))/R₂(1).

The first segment starts by opening switches S₃ and S₅. The capacitor C₂ is charged by the current I₁, giving a positive-going ramp output voltage. Negative-going segments can be produced by turning switches S₃ and S₄ ON, and turning S₁ and S₂ OFF. In this embodiment, a ramp waveform generator produces substantially constant slope within each segment regardless of the load condition.

It is noted that since a zero crossing detector must detect the zero crossing preferably continuously, a continuous-time voltage comparator can be used as a zero-crossing detector.

FIG. 20 illustrates an embodiment of a continuous-time voltage comparator to be used in the present invention. As illustrated in FIG. 20, the continuous-time comparator includes a first amplifier stage 505 that amplifies the difference between the two input voltages v₁ and v₂.

The first amplifier stage 505 includes a clamping circuit 5050, as illustrated in FIG. 21, wherein the clamping circuit 5050 includes transistors M₈ and M₉. The clamping circuit 5050 limits the output voltage swing to shorten the delay. It is noted that a similar clamping circuit can be employed in other amplifier stages if desired.

As illustrated in FIG. 20, the output of the amplifier 505 is connected to a bandwidth select circuit 600. The output of the first amplifier stage is further amplified by a second amplifier stage 510. A latch circuit 700 generates logic level output. It is noted that a Schmitt trigger-type circuit can be used as a continuous-time latch.

The bandwidth select circuit 600 provides different bandwidths depending on different segments of the ramp waveform. The zero crossing detection during the first segment can be a coarse detection, as the zero crossing detection during the first segment does not have to be very accurate. However, since the detection is made while the input to the detector is changing rapidly, the detection needs to be fast.

On the other hand, the zero crossing detection during the last segment in a given cycle determines the accuracy of the output voltage. Therefore, the bandwidth during the first segment can be designed to be high and progressively lower for detection in successive segments to reduce noise for accurate zero crossing detection.

Since the mean-square value of noise is approximately proportional to bandwidth, the bandwidth is made lower during the later segments by the bandwidth select circuit. The bandwidth selection is made at the output of the first amplifier stage 505 by the input signal BW, as illustrated by FIG. 21. Alternatively, the bandwidth selection can be made at the output of the second amplifier stage 510 as illustrated by FIG. 22. Also, it is noted that the number of amplifier stages is increased in a high resolution operation.

An alternative method of bandwidth change is to control the bias currents I_(bias1) and/or I_(bias2) by the zero crossing detector instead of switching in or out the band-limiting capacitors C₁ and C₂. The higher the bias currents of a stage, the higher the bandwidth of that stage becomes.

As noted above, FIG. 21 illustrates another embodiment of a continuous-time voltage comparator to be used in the present invention. As illustrated in FIG. 21, a first amplifier stage 505 includes transistors M₁ and M₂ and resistors R₁ and R₂. A second amplifier stage 510 includes transistors M₅ and M₆ and resistors R₃ and R₄.

It is noted that a clamping circuit that limits the output voltage swing can be employed to shorten the delay in the amplifier stages.

Since the effect of the noise from the second amplifier stage 510 is reduced by the voltage gain of the first stage 505, a second stage bias current I_(bias2) can be made lower than a first stage bias current I_(bias1) in order to reduce power consumption.

As illustrated in FIG. 21, a latch includes transistors M₇ and M₈ and inverters Inv₁ and Inv₂. A bandwidth select circuit 600 includes switches M₃ and M₄ and capacitors C₁ and C₂

For the zero crossing detection during the first segment where high speed is desired, the switches M₃ and M₄ are kept OFF. The bandwidth of the first amplifier stage 505 is determined by the resistors R₁ and R₂ and parasitic capacitance at the output of the first stage 505.

For the zero crossing detection during the segments where low noise is necessary, the switches M₃ are M₄ are turned ON. The added capacitance from C₁ and C₂ at the outputs of the first stage 505 reduces the bandwidth, lowering the noise.

With nominally matched transistors and resistors, the detection threshold is zero. However, by varying either R₁ or R₂, the detection threshold can be varied to cancel the effect of overshoot in each segment of the ramp.

FIG. 22 illustrates another embodiment of a continuous-time voltage comparator to be used in the present invention. As illustrated in FIG. 22, a first amplifier stage 505 includes transistors M₁ and M₂ and resistors R₁ and R₂. A second amplifier stage 510 includes transistors M₅ and M₆ and resistors R₃ and R₄.

It is noted that a clamping circuit that limits the output voltage swing can be employed to shorten the delay in the amplifier stages.

Since the effect of the noise from the second amplifier stage 510 is reduced by the voltage gain of the first stage 505, a second stage bias current I_(bias2) can be made lower than a first stage bias current I_(bias1) in order to reduce power consumption.

As illustrated in FIG. 22, a latch includes transistors M₇ and M₈ and inverters Inv₁ and Inv₂. A bandwidth select circuit includes switches M₃ and M₄ and capacitors C₁ and C₂.

For the zero crossing detection during the first segment where high speed is desired, the switches M₃ and M₄ are kept OFF. The bandwidth of the first amplifier stage 505 is determined by the resistors R₁ and R₂ and parasitic capacitance at the output of the first stage 505.

For the zero crossing detection during the segments where low noise is necessary, the switches M₃ are M₄ are turned ON. The added capacitance from C₁ and C₂ at the outputs of the second stage 510 reduces the bandwidth, lowering the noise.

With nominally matched transistors and resistors, the detection threshold is zero. However, by varying either R₁ or R₂, the detection threshold can be varied. This is a useful feature to cancel the effect of overshoot in each segment of the ramp.

FIG. 23 illustrates another embodiment of a continuous-time voltage comparator to be used in the present invention. As illustrated in FIG. 23, a first amplifier stage 505 includes M₁, M₂, M₃, and M₄. A second amplifier stage 510 includes transistors M₇, M_(g), M₉, and M₁₀. Bias voltages V_(bias1) and V_(bias2) are chosen to provide appropriate gain in the first and the second amplifier stages.

For small gain, the load transistors M₃, M₄, M₉, and M₁₀ are biased in the triode region. If a large voltage gain is desired from the first stage amplifier 505, the load transistors M₃ and M₄ can be biased in the saturation region. In such cases, the voltage V_(bias1) is controlled by a common-mode feedback circuit.

It is noted that a clamping circuit, which limits the output voltage swing, can be utilized to shorten the delay in the amplifier stages.

Since the effect of the noise from the second amplifier stage 510 is reduced by the voltage gain of the first stage 505, the second stage bias current I_(bias2) can be made lower than the first stage bias current I_(bias1) in order to reduce power consumption.

As illustrated in FIG. 23, a latch includes transistors M₁₁ and M₁₂ and inverters Inv₁ and Inv₂. A bandwidth select circuit includes switches M₅ and M₆ and capacitors C₁ and C₂.

For the zero crossing detection during the first segment where high speed is desired, the switches M₅ are M₆ are kept OFF. The bandwidth of the first amplifier stage 505 is determined by bias voltages V_(bias1) and parasitic capacitance at the output of the first stage.

For the zero crossing detection during the segments where low noise is necessary, the switches M₅ are M₆ are turned ON. The added capacitance from C₁ and C₂ at the outputs of the first stage reduces the bandwidth, lowering the noise. With nominally matched transistors and resistors, the detection threshold is zero. However, by varying the gate voltages on M₃ or M₄ individually, the detection threshold can be varied to cancel the effect of overshoot in each segment of the ramp.

FIG. 24 illustrates another embodiment of a continuous-time voltage comparator to be used in the present invention. As illustrated in FIG. 24, a first amplifier stage 505 includes transistors M₁, M₂, M₃, and M₄. A second amplifier stage 510 includes transistors M₇, M₈, M₉, and M₁₀.

It is noted that a clamping circuit, which limits the output voltage swing, can be utilized to shorten the delay in the amplifier stages.

Bias voltages V_(bias1) and V_(bias2) are controlled together with transistor sizes and bias currents for appropriate gain and bandwidth in the first and the second amplifier stages. For small gain and large bandwidth, the load transistors M₃, M₄, M₉, and M₁₀ are biased in the triode region.

Bandwidth can be reduced and gain made larger by raising one or both of the bias voltages. Alternatively, bandwidth can be reduced and gain made larger by making bias current I_(bias1) lower. If larger voltage gain and correspondingly low bandwidth is desired, the load transistors M₃ and M₄ can be biased in the saturation region. In such cases, the voltage V_(bias1) is controlled by a common-mode feedback circuit.

With nominally matched transistors and resistors, the detection threshold is zero. However, by varying the gate voltages on M₃ or M₄ individually, the detection threshold can be varied to cancel the effect of overshoot in each segment of the ramp.

FIG. 25 illustrates another embodiment of a continuous-time voltage comparator with offset cancellation to be used in the present invention. As illustrated in FIG. 25, a first amplifier stage 505 includes transistors M₁ and M₂ and resistors R₁ and R₂. A second amplifier stage 510 includes transistors M₅ and M₆ and resistors R₃ and R₄.

It is noted that a clamping circuit, which limits the output voltage swing, can be utilized to shorten the delay in the amplifier stages.

Since the effect of the noise from the second amplifier stage 510 is reduced by the voltage gain of the first stage 505, a second stage bias current I_(bias2) can be made lower than a first stage bias current I_(bias1) in order to reduce power consumption.

As illustrated in FIG. 25, a latch includes transistors M₇ and M₈ and inverters Inv₁ and Inv₂. A bandwidth select circuits include switches M₃ and M₄ and capacitors C₁ and C₂.

During the offset cancellation phase, switches M₃ and M₄ are closed so that the system common-mode voltage v_(CM) can be applied to inputs, v₁ and v₂, and the desired threshold v_(th) is added to the upper input v₁. The offset of the first stage amplifier 505 plus the desired threshold v_(th) is amplified and output from the first stage 505. This voltage is differentially sampled on capacitors C₁ and C₂.

Next, switches M₃ and M₄ are opened, and the comparator operates normally. The voltage sampled on C₁ and C₂ counteracts with the offset in the first amplified stage canceling the effect and provides accurate detection threshold of v_(th).

FIG. 26 illustrates another embodiment of a continuous-time voltage comparator with offset cancellation to be used in the present invention. As illustrated in FIG. 26, an offset cancellation is included to cancel the offset voltage due to device mismatches in the first stage amplifier.

It is noted that similar offset cancellation method can be applied to other amplifier stages if greater precision is desired.

FIG. 26 illustrates an open loop offset cancellation method; however, it is noted that other methods of offset cancellation can be utilized

During the offset cancellation phase, switches S₁ and S₂ are closed so that the inputs to the amplifiers 505 and 510 are pulled to ground. The offset of the first stage amplifier 505 is amplified and shows up at the output of the first stage. This voltage is sampled on a capacitor C. Next, switches S₁ and S₂ are opened, and the comparator operates normally. The voltage sampled on C counteracts with the offset in the first amplified stage canceling the effect of the offset.

FIG. 27 illustrates another embodiment of a zero crossing detector. As illustrated in FIG. 27, a zero crossing detector includes plurality of threshold voltages. The circuit in FIG. 27 provides two different detection thresholds although more thresholds can be introduced as desired.

A first voltage V_(th1) is first applied to the input of the first stage amplifier 505 by throwing the switch S₁ to a first position. Switches S₂ and S₄ are closed to sample the output voltage of the first stage amplifier 505. The switch S₄ is first opened, and then S₂ is opened.

Next, a second voltage V_(th2) is first applied to the input of the first stage amplifier 505 by throwing the switch S₁ to a second position. Switches S₃ and S₄ are closed to sample the output voltage of the first stage amplifier 505. The switch S₄ is first opened, and then S₃ is opened.

The zero crossing detector is then operated normally by throwing the switch S₁ to the third (open) position. With S₂ closed, the first detection threshold V_(th1) is selected, and with S₃ closed, the second detection threshold V_(th2) is selected. Each detection threshold can be adjusted to cancel the effect of overshoot during each segment.

FIG. 28 illustrates another embodiment of a zero crossing detector. As illustrated in FIG. 28, a zero crossing detector includes a plurality of zero-crossing detectors. A first zero-crossing detector is preferably fast, but the first zero-crossing detector is not necessarily very accurate.

On the other hand, a second zero crossing detector is preferably low noise and high accuracy, but the second zero crossing detector is not necessarily fast.

The first detector is activated during the first segment, whereas the second detector is activated in the second segment. The detection threshold of each detector can be adjusted to cancel the effect of overshoot during each segment.

As illustrated in FIG. 28, the first zero crossing detector includes a first stage amplifier 505, a second stage amplifier 510, and a first latch 700. The second zero crossing detector includes a third amplifier 515 and a second latch 710.

For fast speed, the third amplifier 515 is a Schmitt trigger type with positive feedback. The detection thresholds for each zero crossing detector are optimized separately to cancel the effect of the overshoot. The bandwidth of the first detector is made lower than that of the second detector for lower noise.

As previously noted, in conventional op-amp based circuits, linearity is obtained by large open-loop gain in the op-amp. The large open-loop gain requirement in the op-amp causes conventional op-amp based circuits to be power and area inefficient. Moreover, the large open-loop gain requirement in the op-amp presents difficulty with respect to deep-submicron fabrication of conventional op-amp based circuits.

In zero-crossing based circuits, the linearity depends on the linearity of the ramp waveform. While moderate performance levels have been demonstrated from a non-linearized ramp, effective ramp linearization enables high-accuracy, high speed circuits at very low power consumption. More specifically, an approximate 1% nonlinearity is sufficient for 12 b accuracy in a typical zero-crossing based circuit up to a moderately high speed (.about.100 MS/s). For higher resolution or at very high speed, however, better linearity is required in the ramp.

FIG. 29 illustrates an example of an embodiment of a constant slope ramp circuit. During the preset phase of the zero-crossing based circuit operation, the transistor M₂ is turned ON, presetting the output voltage to V_(DD). As Illustrated in FIG. 29, the switch S.₁ connects the capacitor C to a fixed voltage V_(bias) in order to precharge the capacitor C to an appropriate start voltage. Any other method of initializing the voltage across the capacitor C may be used.

During the charge transfer phase, the switch S₁ and the switch M₂ are turned OFF. The transistor M₁ behaves as a dynamically biased common-source amplifier. If no load is present, the drain current of M₁ is the current I through the capacitor C. Therefore, M₁ behaves as a common-source amplifier with infinite load resistance whose open-loop gain is the intrinsic gain of M₁.

Capacitive loads normally present in zero-crossing based circuits alter the drain current through M₁ to

$I_{DS} = {\frac{C_{L} + C}{C}I}$ where C_(L) is the total effective load capacitance.

Typically, the feedback capacitor C is made much smaller than C_(L) in order to minimize the increase in power consumption.

Since the open-loop gain of the common-source amplifier M₁ is reasonable high, the circuit consisting of the amplifier and the feedback capacitor C behaves as an integrator. With constant input current I, the output voltage V_(out) is a ramp waveform with a constant slope.

The linearity of the ramp is improved approximately by a factor of the intrinsic gain of M₁. Thus, a factor of 30-50 improvement in linearity is easily realized.

As the amplifier is dynamically biased by the displacement current that already exists in the normal zero-crossing based circuit operation, it does not require additional DC bias current, thus does not increase power consumption

Although not illustrated in FIG. 29, the ramp circuit of FIG. 29 must drive two sets of capacitors, which are reconfigured, during the subsequent sampling phase.

FIG. 30 shows an example of a 1-bit per stage pipeline analog-to-digital converter with a conventional ramp circuit where the ramp circuit drives two sets of capacitors (capacitors C₁₁ and C₁₂ and capacitors C₂₁ and C₂₂). For simplicity, only the first two stages of the pipeline are shown.

Capacitors C₁₁ and C₁₂ are first stage capacitors, and capacitors C₂₁ and C₂₂ are second stage capacitors. Switches S₁₁, S₁₂, and S₁₃ configure the first stage capacitors C₁₁ and C₁₂ between the charge transfer phase and the sampling phase as shown in FIGS. 31 and 32. Similarly, switches S₂₁, S₂₂, and S₂₃ configure the second stage capacitors C₂₁ and C₂₂.

For example, during the charge transfer phase of the first stage, as shown in FIG. 31, both switch S₁₁ and switch S₁₂ are thrown to the top position to connect the reference voltage V_(ref) to capacitor C₁₁ and the output from ramp circuit 810 to capacitor C₁₂. During the sampling phase of the first stage, as shown in FIG. 32, both switch S₁₁ and switch S₁₂ are thrown to the bottom position to connect the input voltage V._(in) to capacitor C₁₁ and capacitor C₁₂.

FIGS. 31 and 32 show capacitor configurations during the operation of zero-crossing based circuits wherein FIG. 31 shows a charge-transfer phase, and FIG. 32 shows a sampling phase.

As shown in FIG. 31, the ramp waveform, from ramp circuit 800, is applied to series-connected C_(H) and C₁₂, and parallel-connected C₂₁ and C₂₂ simultaneously.

the sampling phase, as illustrated in FIG. 32, the capacitors C₁₁ and C₁₂ are separated and reconfigured, which requires series switches S₁₂, S₂₁, and S₂₂, as shown in FIG. 30, between the capacitors C₁₂, C₂₁, and C₂₂ and the ramp circuit 800 of FIG. 30. The finite ON-resistance of the series switches S₁₂, S₂₁, and S₂₂ can cause nonlinearity in the resulting zero-crossing based circuits.

In another embodiment, as illustrated in FIG. 33, the ramp circuit 8000 has multiple outputs (V_(out1), V_(out2), and V_(out3)), thereby eliminating the need for the series switches S₁₂, S₂₁, and S₂₂ of FIG. 30 and the associated nonlinearity.

In the ramp circuit 8000 of FIG. 33, the series-connected capacitors C₁₁ and C₁₂ of FIG. 30 are driven by the output v_(out1) while the parallel connected capacitors C₂₁ and C₂₂ are driven by separate outputs v_(out2) and v_(out3). The transistors (M₁, M₂, and M₃) are scaled according to their load capacitance.

The outputs v_(out1) v_(out2), and v_(out3) are nominally identical ramps. Any mismatch between outputs v_(out1) v_(out2), and v_(out3) are due to transistor/capacitor mismatch is equalized by the shorting switches S₄ and S₅.

The multiple output ramp circuit in FIG. 33 allows direct connection of capacitors to respective outputs v_(out1), v_(out2), and v_(out3) without requiring series switches.

During the sampling phase, the outputs of the ramp circuit 8000 are tri-stated, by turning OFF transistors M₂ and M₃, permitting reconfiguration of the capacitors as necessary for that phase.

FIG. 34 illustrates an example of a 1-bit per stage pipeline analog-to-digital converter with multiple output ramp circuits (8100 and 8200). For clarity of illustration, the shorting switches S₄ and S₅ are shown outside the ramp circuit 8100. It is noted that these shorting switches S₄ and S₅ may be components of the ramp circuit 8100.

The capacitors C₁₂, C₂₁, and C₂₂ and the outputs v_(out1), v_(out2), and v_(out3) of the ramp circuit 8100 are directly connected without series switches S₁₂, S₂₁, and S₂₂ of FIG. 30. The outputs v_(out) 1, v_(out) 2, and v_(out) 3 are nominally identical ramps. Any mismatch between outputs V_(out1), V_(out2), and v_(out3) due to transistor/capacitor mismatch is equalized by the shorting switches S₄ and S₅.

With respect to FIG. 34, during the charge transfer phase of the first stage, switch S₁₁ is thrown to connect the reference voltage V_(ref) to capacitor C₁₁, and switch S₁₄ is opened so that the output from ramp circuit 8100 is connected to capacitor C₁₂. Moreover, during the charge transfer phase of the second stage, switch S₂₄ is thrown to connect the reference voltage V_(ref) to capacitor C₂₁, and the output v_(out)1 from ramp circuit 8200 is connected to capacitor C₂₂.

Again with respect to FIG. 34, during the sampling phase of the first stage both switch S₁₁ and switch S₁₄ are thrown to connect the input voltage v_(in) to capacitor C₁₁ and capacitor C₁₂. Furthermore, during the sampling phase of the second stage the outputs of the ramp circuit 8200 are tri-stated.

In summary, a switched capacitor circuit may include a level-crossing detector to generate a level-crossing detection signal when an input signal crosses a predetermined voltage level; a first stage set of capacitors operatively coupled to the level-crossing detector; a ramp circuit operatively coupled to the first stage set of capacitors; and a second stage set of capacitors operatively coupled to the first stage set of capacitors and the ramp circuit.

The ramp circuit may include multiple outputs, a first output being connected to one of the first stage set of capacitors during a charge-transfer phase and a second output being connected to one of the second stage set of capacitors during the charge-transfer phase. The multiple outputs of the ramp circuit may be tri-stated during a sampling phase.

The ramp circuit may include a variable current source, a voltage bias source, and/or a set of shorting switches.

A switched capacitor circuit may include a level-crossing detector to generate a level-crossing detection signal when an input signal crosses a predetermined voltage level; a first stage set of capacitors operatively coupled to the level-crossing detector; a first ramp circuit operatively coupled to the first stage set of capacitors; a second stage set of capacitors operatively coupled to the first stage set of capacitors and the ramp circuit; and a second ramp circuit operatively coupled to the second stage set of capacitors. The first ramp circuit is connected to one of the first stage set of capacitors during a charge-transfer phase. The second ramp circuit is connected to one of the second stage set of capacitors during the charge-transfer phase. The first ramp circuit is connected to one of the second stage set of capacitors during a sampling phase.

The ramp circuit may include a variable current source, a voltage bias source, and/or a set of shorting switches.

A multiple output constant slope ramp circuit may include a variable current source; first, second, and third transistors, operatively connected to the variable current source wherein the first transistor provides a first output voltage, the second transistor provides a second output voltage, and the third transistor provides a third output voltage. The multiple output constant slope ramp circuit may further include a capacitor operatively connected to the first transistor and the variable current source; a first shorting switch operatively connected between the first transistor and the second transistor; and a second shorting switch operatively connected between the second transistor and the third transistor.

The multiple output constant slope ramp circuit may also include a voltage bias source; a switch operatively connected between the voltage bias source and the capacitor; a voltage source; and/or a fourth transistor operatively connected between the voltage source and the first transistor.

While various examples and embodiments have been shown and described, it will be appreciated by those skilled in the art that the spirit and scope of the concepts discussed above are not limited to the specific description and drawings herein, but extend to various modifications and changes. 

I claim:
 1. A switched capacitor circuit, comprising: a level-crossing detector to generate a level-crossing detection signal when an input signal crosses a first predetermined voltage; a first stage set of capacitors coupled to the level-crossing detector; a ramp circuit directly coupled to the first stage set of capacitors; and a second stage set of capacitors coupled to the first stage set of capacitors and the ramp circuit, wherein an output portion of the ramp circuit comprises: a first transistor, a feedback capacitor configured to couple to the first transistor, the feedback capacitor enables a negative feedback loop, and a second transistor coupled to the feedback capacitor and the first transistor, wherein the second transistor enables the output of the ramp circuit to be preset at a second predetermined voltage.
 2. The circuit as claimed in claim 1, wherein the ramp circuit comprises a current source.
 3. The circuit as claimed in claim 1, wherein the ramp circuit comprises a voltage bias source.
 4. A switched capacitor circuit comprising: a level-crossing detector to generate a level-crossing detection signal when an input signal crosses a predetermined voltage level; a first stage set of capacitors coupled to the level-crossing detector; a ramp circuit coupled to the first stage set of capacitors; and a second stage set of capacitors coupled to the first stage set of capacitors and the ramp circuit, wherein the ramp circuit comprises: multiple outputs, a first of the multiple outputs being coupled to one of the first stage set of capacitors during a charge-transfer phase and a second of the multiple outputs being coupled to one of the second stage set of capacitors during the charge-transfer phase, a first transistor, a feedback capacitor configured to couple to the first transistor, wherein the feedback capacitor enables a negative feedback loop, and a second transistor coupled to the feedback capacitor and the first transistor, wherein the second transistor enables the multiple outputs of the ramp circuit to be preset at a predetermined voltage.
 5. The circuit as claimed in claim 4, wherein the multiple outputs of the ramp circuit are tri-stated during a sampling phase.
 6. The circuit as claimed in claim 4, wherein the ramp circuit comprises a current source.
 7. The circuit as claimed in claim 4, wherein the ramp circuit comprises a voltage bias source.
 8. The circuit as claimed in claim 4, wherein the ramp circuit comprises a plurality of shorting switches.
 9. A switched capacitor circuit, comprising: a level-crossing detector to generate a level-crossing detection signal when an input signal crosses a predetermined voltage level; a first stage set of capacitors coupled to the level-crossing detector; a first ramp circuit coupled to the first stage set of capacitors; a second stage set of capacitors coupled to the first stage set of capacitors and the first ramp circuit; and a second ramp circuit coupled to the second stage set of capacitors; wherein the first ramp circuit is coupled to one of the first stage set of capacitors during a charge-transfer phase, the second ramp circuit is coupled to one of the second stage set of capacitors during the charge-transfer phase; and the first ramp circuit is coupled to one of the second stage set of capacitors during a sampling phase.
 10. The circuit as claimed in claim 9, wherein the ramp circuit comprises a current source.
 11. The circuit as claimed in claim 9, wherein the ramp circuit comprises a voltage bias source.
 12. The circuit as claimed in claim 9, wherein the ramp circuit comprises a plurality of shorting switches. 